1. The Field of the Invention
The present invention relates generally to electron emitters. More particularly, the present invention relates to thermionic emission of electrons for x-ray generation.
2. The Relevant Technology
The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
An x-ray tube typically includes a vacuum enclosure that contains a cathode assembly and an anode assembly. The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing may be covered with a shielding layer (composed of, for example, lead or a similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits. The cathode assembly generally consists of a metallic cathode head assembly and a source of electrons highly energized for generating x-rays. The anode assembly, which is generally manufactured from a refractory metal such as tungsten, includes a target surface that is oriented to receive electrons emitted by the cathode assembly.
During operation of the x-ray tube, the cathode is charged with a heating current that causes electrons to “boil” off the electron source by the process of thermionic emission. An electric potential on the order of about 4 kV to over about 200 kV is applied between the cathode and the anode in order to accelerate electrons boiled off the electron source toward the target surface of the anode assembly. X-rays are generated when the highly accelerated electrons strike the target.
Most of the electrons that strike the anode dissipate their energy in the form of heat. Some electrons, however, interact with the atoms that make up the target and generate x-rays. The wavelength of the x-rays produced depends in large part on the type of material used to form the anode surface. X-rays are generally produced on the anode surface through two separate phenomena. In the first, the electrons that strike the cathode carry sufficient energy to “excite” or eject electrons from the inner orbitals of the atoms that make up the target. The material emits x-rays having a characteristic wavelength when the vacancies created by the “excited” or ejected electrons are filled by electrons from outer orbitals. In the second process, some of the electrons from the cathode interact with the atoms of the target element such that the electrons are decelerated around them. These decelerating interactions are converted into x-rays by conservation of momentum through a process called bremstrahlung. Some of the x-rays that are produced by these processes ultimately exit the x-ray tube through a window of the x-ray tube, and interact with a patient, a material sample, or another object.
It is generally desirable to maximize x-ray flux (i.e., the number of x-ray photons emitted per unit time) and minimize the extent of the x-ray source on the anode surface in order to produce a tightly controlled x-ray beam source. These goals are not always compatible.
It is generally acknowledged that diagnostic image quality is at least partially a function of the number of electrons that impinge upon the target surface of the target anode. In general, more electrons results in higher x-ray flux, which in turn results in x-ray images with higher contrast (i.e., higher quality). The performance of a particular emitter can thus be evaluated in terms of the efficiency of that emitter, where the efficiency of the emitter is defined as the number of electrons impinging upon the target surface of the target anode, i.e., the perveance of the emitter, as a percentage of the total number of electrons discharged by the emitter. In general then, image contrast or quality improves as the efficiency of the emitter increases.
While the quality of the images generated by an x-ray device is to a large extent a function of emitter efficiency, it is also well understood that the quality of diagnostic images additionally depends on the pattern, or focal spot, created by the emitted beam of electrons on the target surface of the target anode. In general, a smaller focal spot produces better quality x-ray images. This phenomenon can readily be analogized to the shadows produced by a visual light source. For example, the shadows cast by a sharp light source (e.g., a point source such as a laser) are themselves sharp, while the shadows cast by a poorly defined light source (e.g., fluorescent office lights) are themselves poorly defined and diffuse. The same is true of the shadows cast by the x-rays that are transmitted and absorbed as x-rays pass through a subject.
Another important consideration in the design of x-ray devices is the physical limits of the anode. As mentioned above, x-rays are generated when electrons from the electron beam strike the anode surface. Nevertheless, the fact that most of the electrons that impinge on the anode surface dissipate their energy in the form of heat can sometimes lead to anode overheating and failure if the electron beam flux is very high and/or the electron beam is relatively intense and very tightly focused on the anode surface. This is especially true in the 60-150 kilovolt operating range typical of diagnostic medical x-ray devices.
Based on the foregoing discussion, it should be generally understood that it is desirable to have an electron emitter that maximizes electron beam flux for optimal x-ray image contrast, while simultaneously providing for the smallest focal area allowable within the physical limits of the anode.